Method and means for continuous hot-dip aluminizing of materials



June 9, 1959 H. E. LINDEN ET AL v 2,890,134

METHOD AND MEANS FOR CONTINUOUS HOT-DIP ALUMINIZING OF MATERIALS Filed sept. 21, 195e United States Patent @thee 2,890,134 Patented J une 9, `1 959 METHOD AND MEANS FOR CONTINUOUS HOT-DIP ALUMINIZING OF MATERIALS Herbert E. Linden and David W. Mitchell, Los Angeles, lCalif., assignors to American Mollerizing Corporation, Beverly Hills, Calif., a corporation of Nevada Application September 21, 1956, Serial No. 611,333

Claims. (Cl. 117-51) This invention relates generally to the coating of materials, and specifically relates to a method and means for continuously coating metallic materials with molten aluminum or aluminum alloys.

In the past, typical hot-dip processes for the alumini- Zation of metals have consisted in dipping the metal to be coated into a molten salt bath where it is cleaned and otherwise pretreated and brought to a high aluminizing temperature, and withdrawing the pretreated metal object directly into an overlying aluminum layer to be thereby coated. Such hot-dip processes, while advantageous in many respects, such as in the elimination of oxidation during the actual coating, and in the formation of an integrally bonded coating, suffer from the disadvantage that the resulting coated product is sometimes highly rittle in nature, especially in thin pieces of material, such as strip, wire, or sheet.

The brittleness of the product formed is found to be due to the diffusion of the coating metal within the base metal during the coating operation, the diffusion causing a coating metal-base metal interfacial alloy to be formed. The alloy itself is a highly brittle material, and if it is of suicient thickness `with respect to the total thickness of the product, its properties will be imparted to the entire product.

It is found that the amount of diffusion of the metals within each other can be controlled to a great degree by controlling the temperature at which the aluminizing, that is, the coating of the base metal with aluminum, or its alloys, takes place. Generally speaking, the higher the temperature of the aluminum coating layer, the greater the diffusion, and the more ibrittle the product, other factors being equal.

The lowest temperature at which hot-dip aluminizing occurs is necessarily the melting point of the coating material, since in order for the desired bonded coating to occur, the coating material must be maintained in the liquid state. Ideally, therefore, the melting point of the particular aluminum or aluminum alloy that is to be used as the coating metal, is the lowest temperature at which aluminization can take place.

It is desirable to maintain the aluminum in a molten state, by means of a salt bath, by direct conduction of heat therefrom `for a number of reasons, described in detail in U.S. Patent No. 2,315,725, entitled Process for Metallization, Especially Aluminization of Iron Articles to G. A, Moller. Thus, in order to maintain the aluminum coating metal at temperatures just above its melting point, the temperature of the salt lbath itself must be maintained, generally speaking, -50 above the melting point of the coating bath. Since pure aluminum melts at 1218 F., and the alloys at a lower temperature, the maximum temperature of the salt bath -Would theoretically be about 1235-1260 F. Such a low temperature in the salt bath is, however, disadvantageous for a number of reasons.

One purpose of the salt bath is to clean the metal surface of oxides and other foreign matter immediately prior to its insertion in the coating metal. Such cleaning action proceeds most advantageously at much higher temperatures than l260 F., and proceeds preferably most advantageously in the region of 1400 to 1600 F. Secondly, if a steel is to be coated, it is frequently desirable also to heat-treat it, and the heat treating of steel requires that the steel be first completely transformed into a stable austenitic structure. The minimum theoretical temperature for this transformation to occur in steel is 1333 F. However, the practical minimum temperature at which heat treating occurs in steel is as much as several hundred degrees F. above the minimum theoretical temperature, if complete and rapid formation of the desired hardened structure is to be obtained.

While the salt bath may be maintained in the region of 1600 F., thus maintaining the aluminum coating layer at substantially the same temperature, as is done in the Moller process, described in the above-identified patent, such a process is not desirable inasmuch as the alloying action between the base metal and the aluminum coating metal proceeds at a much higher rate, thereby forming a highly brittle product. Conversely, while the salt bath may be maintained just above the melting point of the aluminum coating layer, such a process is not advantageous inasmuch as the cleaning and heat-treating of ferrous and other metals is inhibited.

Bearing in mind the foregoing discussion, it is a major object of the present invention to provide means and a method for continuously aluminizing metals whereby an improved ductile product is produced.

lt is another object of the present invention to provide means and a method for rapid and continuous hot-dip aluminizing of continuous metallic materials whereby a heat treated ductile aluminized metal is produced.

It is also an object of the present invention to provide a means and a method for the hot-dip aluminizing of metals whereby the aluminum coating metal-base metal interface is minimized and the product produced is heat treated.

Another object of the present invention is to provide means and a method for the hot-dip aluminizing of metals whereby the metal to be coated is pretreated at a temperature substantially above the temperature of austenite formation, the base metal being then coated with aluminum at a substantially lower temperature so that the aluminized metal produced is ductile.

These, and other objects, of the present invention will become apparent by referring to the following description, and to the accompanying drawings, in which:

Figure l is a perspective view of an apparatus for the continuous hot-dip aluminizing of thin materials, one wall of said apparatus being removed to reveal the interior thereof; and

Figure 2 is a cross-section along the line 2 2 of Figure 1, showing the arrangement of the various materials contained therein.

Figure 3 is a cross-section of a modification of the coating compartment of the apparatus shown in Figure 1.

In general, our means and method for continuously aluminizing continuous thin metals, such as steel sheet, strip or wire, depends upon the maintenance of a temperature differential between interconnected pretreating compartments and a coating compartment in the same furnace. 'I'he base metal to be coated is subjected to the highest practical temperature in a rst pretreating compartment, so that it wil-l be readily cleaned and deoxidized therein, then passed directly into a second interconnecting, lower temperatured compartment, and then with drawn into an interconnecting still lower temperatured coating compartment, the resulting alloying action being minimized due to the maintenance of the lowest possible coating temperature in the coating compartment.

An apparatus which is advantageously utlized in the process of the present invention is shown in Figures 1 and 2.

Referring now to the drawing, a preferably rectangular furnace is designated by the numeral and comprises a crucible 11 divided into three sections or compartments, the right-hand or pretreating section 12, the intermediate pretreating section 13, and the left-hand or coating section 14, by right-hand and left-hand partition Walls 16, 16a respectively. The partition walls 16, 16a, are generally transversely affixed to the side walls 17 of the crucible 11.

Each of the pretreatiug compartments 12 and 13 are preferably substantially longer than the coating compartment 14 so that the base metal to be coated may be passed very rapidly through them and still attain a temperature suitable for a high degree of cleaning, deoxidation and subsequent heat treatment. Thus, for example, if the interior dimensions of the Crucible 11 are 2 feet deep, 2 feet wide, and 30 feet long, the pretreating compartments 12, 13 occupy a volume of from 20 to 29.5 feet long by 2 feet wide by 2 feet deep, and the coating compartment 14 comprises a volume of about 0.5 to 1 X 2 x 2 feet. The total length of the pretreating compartments 12, 13 vary between about 20 to 29.5 feet in length, depending upon such factors as the composition of the salt bath used, the condition of the base metal, and the desired end condition of the resulting product, the length of each of the individual pretreating compartment varying between approximately 10 and 20 feet depending on these same factors. The crucible 11 of the furnace 10 is constructed of an inner liner 18 of suitable heat and chemical resistant material, such as a high-density alumina refractory, and a backing layer of a second refractory material such as standard refractory brick 20, suitably reinforced, as by an outer steel casing (not shown).

The partition Walls 16, 16a are also provided with a heat and chemical resistant liner 1S on each side face thereof, the liners being separated by an insulating refractory layer 21 so that the temperature in each of the compartments 12, 13 and 14 is readily maintainable at substantially different levels.

Each compartment 12, 13 and 14 is provided with a number of heating electrodes imbedded in the side walls 17 of each of the compartments 12, 13 and 14, respectively, and are connected to a suitable electrical source (not shown) whereby the contents of each compartment are heated individually.

The insulated partition walls 16, 16a are provided with a plurality of openings or passages 22 which interconnect each of the compartments 12, 13 and 14, the diameter of the openings being preferably only slightly larger than the diameter of the metal to be passed therethrough. Thus, if a continuous wire 24 is to be aluminized, the passages 22 have a diameter slightly greater than the diameter of the wire to be coated, and are spaced from each other a suicient amount to prevent any interference of the wires therebetween. If continuous thin, elongated materials, such as strip or sheet, are to be continuously coated, the passages 22 are correspondingly elongated and made slightly wider than the strips or sheets to be passed therethrough.

In order to facilitate the threading of the continuous wire 24 through the interconnecting passages 22, the partition walls 16, 16a are preferably provided with a number of removable sections 26, each section being stably mounted within the partition wall by any suitable means, as for example, by a tongue and groove construction. These sections 26 are vertically mounted above each of the interconnecting passages 22, and each is preferably provided with an internal heating means, such as an elongated heating electrode 27 adapted to be connected to a suitable source of electric current (not shown), for the purpose of unfreezing, that is melting the salt (to be described) which would otherwise prevent the removal of the removable sections 26 from the partition walls 16, 16a. If continuous thin, elongated materials are to be coated, such as strip or sheet, the removable sections 26 are correspondingly elongated to allow the insertion of such continuous materials.

It is also possible that individual small pieces, hooked onto a conveyor chain can be coated utilizing our method and apparatus, the passages and the removable sections of the apparatus being modified to accommodate the pieces passed therethrough. The term continuous materials therefore includes the coating of individual pieced materials axed to a continuous chain.

The interconnecting passages 22, are of the smallest convenient size with respect to the materials to be passed therethrough as has been mentioned, in order that heat transfer between the compartments 12, 13, and 14 be minimized. The temperature in each compartment is thus readily maintainable at substantially different levels.

The interior of each compartment 12, 13 and 14 of the crucible 11 contains a bath of molten salt 31 which preferably completely covers the electrodes 26 in each of the compartments. The molten salt 31 also iills the interconnecting passages 22. The coating section 14 contains a molten aluminum or aluminum alloy layer 32, preferably 1 to 4 inches in thickness, which rests upon the salt layer 31 and is in direct contact therewith. The salt bath 31 and the aluminum coating metal 32 are integral parts of the furnace 10 since the molten materials are replenished as needed, the levels of these materials, therefore, being maintained essentially constant over relatively long periods of time.

The salt bath 31 is preferably composed of the chloride, bromide, iodide, and uoride salts, especially of the alkali and alkaline earth group. Also, the metal fluorides, such as aluminum fluoride and cryolite (sodium aluminum fluoride) are especially advantageous.

The precise combination of salts used is determined primarily by the composition and density of the coating metal and also by the composition and physical condition of the base metal to be coated. For example, if pure aluminum metal is the coating material, the temperature at which it melts is 1218o F. The pretreating bath 31 must, therefore, comprise a combination of salts that is stable above 1218" F. If an aluminum alloy is to be used, however, as the coating material, the temperature of the salt bath need not be maintained as high as with the pure aluminum, since the melting points of the aluminum alloys are generally lower than 1218 F. Hence, a factor in the determination of the composition of the salt bath is the composition of the coating .metal itself.

Generally, if the metal to be coated has a large amount of oxides on its surface, it is advisable to include in the salt bath composition a greater amount of fluorides than would otherwise be necessary, in order to dissolve more of the surface oxides. At other times, the use of fluorides is entirely unnecessary. Hence, the condition of the base metal is a factor entering into a determination of the salt bath composition.

The composition of the salt bath is also determined by the density of the coating metal. It is preferable to support the aluminum coating metal on the salt bath, and therefore the density of the salt bath is a factor entering into the determination of its composition.

Bearing in mind all these factors, specific salt bath compositions that have been employed with excellent results in the coating of metals with pure aluminum and its alloys at salt bath temperatures, ranging between about 1050 to 1650 F., comprise the following:

70-75% barium chloride by weight 20-25% sodium chloride by weight (LO-10% sodium fluoride by Weight 70-75% calcium bromide by weight 20-25% sodium chloride by weight O O-% sodium aluminum fluoride by weight III 70-75% potassium iodide by weight -25% sodium chloride by weight 0.0-10% aluminum uoride by weight 70-75% barium bromide by weight 20-25% sodium chloride by weight 0.0-10% aluminum fluoride by weight Composition I is presently preferred because of the relative cheapncss of the chlorides.

The conveying means for the coating of continuous metallic materials comprises preferably three guide rollers 33, 34 and 34a, the roller 33 being aiiixed to the end wall of the pretreating compartment, the submerged roller 34 being affixed within the coating compartment 14, and the roller 34a aiixed tangentially above said submerged roller 34, to any suitable fixed point. The roller 34a is preferably spaced 10420 feet above the top of the furnace 10 in order that the continuous material that has just been coated may be cooled prior to being bent.

Grooves 36 are circumscribed about the rollers 33, 34, and 34a respectively, a groove on each roller being aligned in the vertical plane passing through the axis of each interconnecting passage 22. Thus, wires 24, being conveyed through the furnace 10, are retained within spaced parallel vertical planes and have no appreciable sideward movement.

Having set forth a preferred embodiment of our coating apparatus, a preferred method of utilizing the process to coat continuous thin materials will now be described.

The temperature of the pretreating section 12 is preferably maintained at approximately 1600 to l650 F., in order to obtain a high degree of cleaning and/or to austenitize the base metal. The temperature of the coating section 14, however, is maintained at a temperature just hot enough to keep the aluminum layer 32 in a molten state. If the coating metal 32 is an aluminum alloy having a melting point less than that of the molten aluminum, the temperature of the coating section 14 is correspondingly lower, and is approximately 1050 to 1225 F.

The intermediate pretreating section has a temperature range of from approximately l333 F. to 1500"l F., that is, intermediate the high temperature of the rst pretreating section 12 and the relatively low temperature of the coating section 14. The presence of the intermediate temperature zone 13 is preferable in order to lower the temperature of the wire 33 as much as possible consistent with high temperature cleaning preparatory to its immersion in the final coating compartment 14, to more readily inhibit heat transfer from compartment to compartment, to aid in the maintenance of a closely controlled temperature in each of the compartments, and also to economize on heating costs.

It is realized that a two-compartment furnace could be employed with satisfactory results in some instance, the pretreating section being maintained at about 1500 to 1650" F., and the coating section maintained between 1050J and 1260 F.

In the process for coating metallic materials the continuous material, for example, a wire 24, is drawn over the roller 33 and into the pretreating section 12, thence diagonally through the molten salt 31, through the interconnecting passage 22, thence into the intermediate temperatured pretreating compartment 13, and into the substantially lower temperatured coating section 14. The wire 24 is then withdrawn vertically upwardly through the salt bath 31 around submerged roller 34 of the coating section 14, through the overlying molten aluminum layer 32, and outwardly over the roller 34a to subsequent process steps, such as rinsing or quenching.

The path of the wire 24 through the pretreating sections is sufficiently long so that the wire will attain substantially the temperatures of each of the pretreating salt baths 31. These temperatures are attained even though the wire is continuously coated at rates of up to several hundred feet per minute.

As the pretreated wire 24 enters the coating section 14, its temperature will be relatively low, and upon reaching the aluminum layer 32, its temperature is decreased to that substantially of the aluminum layer 32 itself. Thus, the alloying action between the base metal and the aluminum is minimized because the coating takes place at the minimum possible temperature, that is, the temperature at which the coating metal melts.

It is thus seen, that while the metal is subjected to an improved cleaning and deoxidation step and also prepared for subsequent heat treating operations by the initial forming of austenite in the pretreating sections 12- and 13, diffusion of the coating and base metals within each other during the coating operations is nevertheless minimized, and a thin aluminum-base metal alloy interface is produced, which renders the entire product ductile.

In some cases it may be desirable to have more than a three-compartment furnace. For example, it may be advantageous to have a number of dilfcrent temperatured intermediate Zones in the furnace 10 placed between an initial high temperatured zone and a relatively low temperatured coating zone.

Referring now to Figure 3, a modification of the coating section is there shown and indicated generally by the numeral 14a. The coating section 14a is relatively enlarged with respect to the coating section 14, the partition wall having been moved somewhat to the right and is indicated generally by the numeral 16h. A baffle 40, aflixed to the side wall 17 of the crucible 11 in the .coating section 14a, terminates above the floor 42 of the 'furnace 10.

A salt bath 31, having one of the compositions previously described, is provided in the coating section 14a, the surface of the salt bath being above the lower edge of the baille 40, and supporting a thin layer of aluminum 32 placed in the narrow left-hand sub-section 44 of the coating compartment 14u.

A submerged roller 46 is pivotally mounted to the baffle 40 and may be fixed in any desired position in the coating section 14a. Thus, when coating material such as strip 50, the roller 46 is placed in the aluminum subsection 44 and so positioned that the strip or wire being fed there around may be drawn vertically upwardly through the aluminum layer 32.

The arrangement of the coating section described and illustrated with reference to Figure 3 is advantageous in many instances, especially Where the roller mechanism 46 is made of a material that is fairly readily attacked by molten aluminum. In such instances, when the submerged roller 46 is to be repaired or handled for any other reason, it can be taken out of the furnace 10 for inspection through the salt bath sub-section 52 of the coating section 14a by merely pivoting upwardly to the right rather than being withdrawn through the aluminum portion 32 of the coating section 14a. Where the submerged roller 46 is made of a special metal alloy or a ceramic refractory material that is highly resistant to molten aluminum, the simpler construction of the coating section 14 of Figures 1 and 2 is preferably employed.

The types of metals that have been aluminized with especially excellent results include the ferrous metals (eg. steel), nickel, cobalt, manganeses, titanium and copper metals and alloys thereof. These metals fall in the transition group of elements, with the exception of titanium and have atomic numbers ranging from 25 for manganese to 29 for copper. number of 22.

While a preferred embodiment of our process and apparatus has been described and illustrated, it is apparent that many modifications and changes may be made therein which lie within the scope of the invention. Therefore, we do not intend to be limited by the specific embodiment described herein, but intend to be limited only by the appended claims.

We claim:

l. A process for the continuous coating of continuous material with a molten coating metal which comprises the steps of: immersing the material to be coated in a iirst molten salt bath having a temperature substantially above the melting temperature of the coating metal; passing said material immediately from said first salt bath into at least one lower-temperatured molten salt bath maintained at a temperature above the melting point of said coating metal; and withdrawing said material from the last of said lower temperatured salt bath directly into a bath of molten coating metal maintained at a temperature substantially equal to its melting point.

2. The process as defined in claim 1, characterized in that the coating metal is selected from the group consisting of aluminum and the aluminum alloys, and further characterized in that all of said salt baths comprise salts selected from the group consisting of alkali and alkaline earth halides and metal fluorides.

3. The process as defined in claim 1, characterized in that the material to be coated attains a temperature substantially equal to the temperature of the rst pretreating salt bath as it passes therethrough, and further characterized in that the material is lowered substantially to the lower temperatures of t-he subsequent salt baths prior to its entry into the molten coating metal.

4. A process for the continuous coating of continuous material with a molten coating metal which comprises the steps of: immersing the material to be coated in a first pretreating molten salt bath having a temperature substantially above the melting temperature of the coating metal; passing said material immediately from said first salt bath into at least two subsequent molten salt baths, the first of which is maintained at a temperature intermediate the melting -point temperature of the coating metal and the temperature of the first bath, and the last of which is maintained at a temperature approximately equal to the melting point temperature of the coating metal; and withdrawing said material from said last salt bath directly into a bath of coating metal, overlying and in direct contact with said last salt bath, to be thereby coated.

5. The process as dened in claim 4 characterized in that the material to be coated attains temperature substantially equal to the temperature of the first pretreating salt bath as it passes therethrough, and further characterized in that the material is lowered substantially to the lower temperatures of the subsequent salt baths prior to its entry into the bath of molten coating metal.

6. A process for the continuous coating of continuous metallic base materials with a molten coating aluminum metal which comprises steps of: immersing the base material to be aluminized in a rst pretreating molten salt bath maintained at a temperature substantially above the melting point of the molten coating metal; passing said material through an interconnecting passage into a second intermediate temperatured pretreating salt bath maintained at a temperature intermediate said first salt bath temperature and the melting point temperature of said coating metal; passing said material from the second bath into a third salt bath maintained at a temperature just sufiicient to maintain said coating metal, overlying and in direct contact therewith, in a molten state; and withdrawing said pretreated material through said lower-temperatured -salt bath, and into and through, said overlying coating metal to be thereby coated.

Titanium has an atomic 7. The process as defined in claim 6 characterized in that all the salt baths comprise salts selected from the group consisting of alkali and alkaline earth bromides, chlorides, iodides, fiuorides, and aluminum-containing fiuorides, said salt baths being of greater density than said coating metal.

8. The process as defined in claim 6 characterized in that the base material attains a temperature substantially equal to the temperature of said first pretreating salt bath, upon passing therethrough, and further characterized in that the base m-aterial attains substantially the lower temperatures of each of the subsequent salt baths prior to its entry into the overlying molten coated metal.

9. The process as detined in claim 6 in which the continuous materials to be coated are selected from the group consisting of iron, cobalt, manganese, copper, nickel, titanium, and their alloys.

l0. The process as defined in claim 6 in which the continuous material to be coated is a ferrous material, and further characterized in that the ferrous material is austenitized as it passes through said rst and second pretreating baths.

ll. The process as defined in claim 10 in which the temperature of the rst pretreating bath is between 1550 and 1650 F the temperature of the second pretreating bath is between 1333 and 1500 F., and the temperature of the third salt bath is between 1050 and l260 F.

l2. A process for the coating of material with a molten coating metal which comprises the steps of: immersing the material to be coated in a first zone having a temperature substantially above the melting temperature of the coating metal; passing said material immediately from said first zone into at least one lower-temperatured zone maintained at a temperature above the melting point of said coating metal; passing said material from the last of said lower-temperatured zones directly into a still lowertemperatured molten salt bath the temperature of said salt bath being maintained between approximately 1050o F. and l260 F.; and withdrawing said material from said salt bath directly into a bath of coating metal, overlying and in direct contact with said salt bath, to be thereby coated.

13. A process for the continuous coating of continuous material with a molten coating metal which comprises the steps of: immersing the material to be coated in a first zone having a temperature substantially above the melting temperature of the coating metal; passing said material immediately from said first zone into at least one lowertemperatured zone maintained at a temperature above the melting point of said coating metal; passing said material from the last of said lower-temperatured zones directly into a still lower-temperatured molten salt bath, the temperature of said salt bath being approximately equal to the melting point temperature of said coating metal; and withdrawing said material from said salt bath directly into a bath of coating metal, overlying and in direct contact with said salt bath, to be thereby coated.

14. A process for the continuous coating of continuous material with a molten coating metal which comprises the steps of: immersing the material to be coated in a first molten salt bath having a temperature substantially above the melting temperature of the coating metal; passing said material immediately from said first salt bath into at least one lower-temperatured molten salt bath maintained at a temperature between 25 F. to 50 F. above the melting point of said coating metal; and withdrawing said material from the last of said lower temperatured salt baths into a bath of molten coating metal maintained at a temperature substantially equal to its melting point` l5. A process for the continuous coating of continuous material with a molten coating metal which comprises the steps of: immersing the material to be coated in a molten salt zone having a temperature substantially above the melting temperature of the coating metal; passing said material into progressively lower-temperatured zones, the lowest temperatured salt zone `being maintained above the melting point of said coating metal; withdrawing said material from the last of said lower-temperatured zones and passing it into a molten coating metal maintained at a temperature substantially equal to its melting point.

16. The process as deined in claim 15 characterized in that the coating metal is selected from the group consisting of aluminum and the aluminum alloys.

17. The process as dened in claim 15 characterized in that said salt bath comprises salts selected from the group consisting of alkali and alkaline earth halides and metal fluorides.

18. The process as defined in claim 15 characterized in that the material to be coated attains a temperature substantially equal to the temperature of the rst zone as it passes therethrough, and further characterized in that the material is lowered substantially to the lower temperatures of the subsequent lower-temperatured zones prior to its entry into the molten coating metal.

19. The process of claim 15 wherein `the temperature of the last of said lower-temperatured zones is maintained at a. temperature from approximately 25 F. to approximately 50 F. above the melting point of said molten coating metal.

20. In apparatus for continuously coating materials, said apparatus including a refractory cmcible having side walls and end walls, at least one insulated partition wall affixed to the side walls of said cnlcible to form a first compartment and a coating compartment, at least one passage formed in a lower portion of said partition wall interconnecting said compartments, means for heating each compartment individually the improvement which comprises: at least one removably insertable wall section provided in each of said partition walls, said Wall sections being positioned above said lower interconnecting passages and having internal heating means; and means for conveying the continuous material into said pretreating compartments, through said interconnecting passage directly into said coating compartment and outwardly from said coating compartment.

References Cited in the le of this patent UNITED STATES PATENTS 2,315,725 Moller Apr. 6, 1943 2,428,523 Marshall Oct. 7, 1947 2,570,906 Alferie Oct. 9, 1951 2,755,542 Boegehold July 24, 1956 2,797,173 Keller June 25, 1957 

6. PROCESS FOR THE CONTINUOUS COATING OF CONTINUOUS METALLIC BASE MATERIALS WITH A MOLTEN COATING ALUMIUM METAL WHICH COMPRISES STEPS OF: IMMERSING THE BASE MATERIAL TO BE ALUMINIZED IN A FIRST PRETEREATING MOLTEN SALT BATH MAINTAINED AT A TEMPERATURE SUBSTANTIALLY ABOVE THE MELTING POINT OF THE MOLTEN COATING METAL; PASSING SAID MATERIAL THROUGH AN INTERCONNECTING PASSAGE INTO A SECOND INTERMEDIATE TEMPERATURED PRETREATING SALT BATH MAINTAINED AT A TEMPERATURE INTERMEDIATE SAID FIRST SALT BATH TEMPERATURE AND THE MELTING POINT TEMPERATURE OF SAID COATING METAL; PASSING SAID MATERIAL FROM THE SECOND 